1. Field
Example embodiments relate to an organic thin film transistor (OTFT) including a fluorine-based polymer thin film and a method of fabricating the same. Other example embodiments relate to an OTFT, including a substrate, a gate electrode, a gate insulating layer, an organic semiconductor layer, and source/drain electrodes, in which a fluorine-based polymer thin film is between the source/drain electrodes and the organic semiconductor layer and a method of fabricating the same.
2. Description of the Related Art
Thin film transistors (TFTs) presently used in displays may be composed mainly of an amorphous silicon semiconductor, a silicon oxide insulating film, and a metal electrode. However, with the recent development of various conductive organic materials, research into OTFTs using organic semiconductors is being conducted all over the world. The OTFT, which was first invented in the 1980s, may be advantageous because it is flexible and may be more easily processed and fabricated, and thus is under study these days for application to display devices, e.g., E-Ink, organic EL and/or LCD.
Because the organic semiconductor, regarded as a novel electronic material, has advantages (e.g., numerous polymer synthesis methods, easier formability into fibers or films, flexibility and/or decreased preparation costs), it is widely applied to functional electronic devices and optical devices. Accordingly, in the OTFT, an organic semiconductor layer may be formed not of amorphous Si but of conductive polymer, and may be thus used as the organic semiconductor of a transistor. Compared to conventional silicon transistors, such an OTFT may be advantageous because the semiconductor layer may be formed through a printing process at atmospheric pressure instead of through CVD using plasma, and all of the fabrication processes may be carried out using a roll-to-roll process on a plastic substrate, if necessary, thus decreasing the cost of fabricating the transistor.
However, the OTFT is disadvantageous because it has decreased charge mobility and increased operating voltage and threshold voltage compared with amorphous silicon TFTS. Charge mobility may be increased to a level of about 0.6 cm2·V−1·sec−1 using pentacene, thus increasing the probability of realizing an OTFT in practice. However, charge mobility may still be unsatisfactory, and an operating voltage of about 100 V or more and a sub-threshold voltage corresponding to at least 50 times the voltage of an amorphous silicon TFT may be required.
In other conventional art, there is an OTFT in which an operating voltage and a threshold voltage are decreased using a high-k insulating film. The gate insulating layer may be formed of inorganic metal oxide, e.g., BaxSr1-xTiO3 (BST; Barium Strontium Titanate), Ta2O5, Y2O3 and/or TiO2, and/or a ferromagnetic insulator, e.g., PbZrxTi1-xO3 (PZT), Bi4Ti3O12, BaMgF4, SrBi2(Ta1-xNbx)2O9, Ba(Zr1-xTix)O3 (BZT), BaTiO3, SrTiO3 and/or Bi4Ti3O12. A gate insulating layer may be formed through CVD, PVD, sputtering and/or sol-gel coating, and may have a k of about 15 or more.
Although the operating voltage of the OTFT is decreased to about −5 V, charge mobility thereof may not be higher than about 0.6 cm2·V−1·sec−1, which is still unsatisfactory. Almost all of the fabrication processes require an increased temperature of about 200° C.˜about 400° C., thus various substrates may not be applied. A general wet process, e.g., simple coating and/or printing, may be difficult to use upon the fabrication of the device.
In other conventional art, there may be an organic insulating film formed of polyimide, benzocyclobutene, or polyacryl. However, the organic insulating film may not exhibit the device properties suitable for substituting for an inorganic insulating film.
With the goal of improving the performance of thin film electronic devices, attempts to use a multilayered gate insulating film having two or more layers have been made. In this regard, a multilayered gate insulating film composed of amorphous silicon nitride and silicon oxide, and a double-layer insulating film may use the same materials as above. Thereby, the electrical insulating properties and crystal quality of the semiconductor layer may increase.
However, the above-mentioned are limited only to inorganic TFTs using amorphous silicon or monocrystal silicon, and may be more difficult to apply to organic semiconductors due to the use of inorganic material.
Recently, application of the OTFT to various devices, including not only LCDs but also devices for driving flexible displays using organic EL, has been attempted. The OTFT may have charge mobility not lower than about 5 cm2·V−1·sec−1, decreased operating voltage and threshold voltage, and improved insulating properties of the insulating film. Especially, the fabrication thereof may be required to be conducted in the all-printing or all-spin on manner on a plastic substrate, in order to simplify the process and reduce the cost.
Accordingly, research into methods of forming an organic gate insulating layer through a simpler process and of increasing charge mobility between the organic gate insulating layer and the organic semiconductor layer thereon has been actively conducted, but there are no satisfactory alternatives. Consequently, the development of an OTFT having a novel structure, which is characterized by ensuring increased charge mobility, having improved electrical insulating properties, lower operating voltage and threshold voltage, and forming an insulating film through a typical wet process, is required.
Moreover, the OTFT may be fabricated to have various structures. However, among these structures, a top contact type OTFT may be undesirable because channel resistance is problematic. A bottom contact type or top gate type OTFT may be undesirable because it has increased contact resistance between source/drain electrodes and an organic semiconductor layer, in addition to the problem of the channel resistance, undesirably deteriorating the performance of the OTFT.